As a so-called direct AC-AC power converter circuit, an indirect matrix converter is well known. The indirect matrix converter comprises a current source converter for performing AC-DC conversion, a voltage source inverter for performing DC-AC conversion, and a DC link for connecting the current source converter and the voltage source inverter.
When the instantaneous space voltage vector modulation is adopted in order to control the voltage source inverter, the modulation factor is applied as a ratio of a line voltage peak value of an AC voltage outputted from the voltage source inverter to a DC voltage inputted to the voltage source inverter.
Japanese Patent Application Laid Open Gazette No. 2000-14200, for example, discloses a technique for obtaining a modulation factor by polar coordinate conversion of a so-called d-axis voltage command and a q-axis voltage command.
In the indirect matrix converter, a voltage (hereinafter, referred to as “link voltage”) applied to a DC link from the current source converter is a DC voltage inputted to the voltage source inverter. Since the indirect matrix converter is a direct AC-AC power converter circuit, however, the indirect matrix converter has the DC link but the DC link does not substantially comprise a smoothing circuit.
Therefore, the DC voltage inputted to the voltage source inverter largely varies with the switching of the current source converter. It is not preferable in terms of accuracy that the modulation factor is obtained without consideration of this variation.
Considering this background, in the field of the direct AC-AC power converter circuit, proposed are the techniques disclosed in Japanese Patent Application Laid Open Gazette Nos. 2009-213252 and 2010-98848.
Japanese Patent Application Laid Open Gazette No. 2009-213252 discloses a technique for detecting an instantaneous value of a DC voltage on the basis of a switching pattern of the current source converter.
Japanese Patent Application Laid Open Gazette No. 2010-98848 discloses a technique for obtaining a DC voltage by using an average value of a first average DC voltage which is obtained by averaging a link voltage in a switching cycle of the current source converter in a section which corresponds to one twelfth of the cycle of an AC voltage inputted to the current source converter.
Other than the above documents, among those discussed below are Japanese Patent Application Laid Open Gazette No. 2007-312589, Japanese Patent Publication No. 4067021, Japanese Patent Application Laid Open Gazette No. 2009-106110 and L. Wei, T. A. Lipo, “A Novel Matrix Converter Topology with Simple Commutation, IEEE ISA2001, vo. 3, pp. 1749-1754, 2001.